1. Field of the Invention
The present invention relates to a failure diagnosis system that diagnoses whether or not a sensitivity adjustment part for adjusting a receiver sensitivity (conversion rate) of an input signal is in failure.
2. Description of the Related Art
In recent years, in response to an increasing demand for environmental protection and a problem about fuel depletion, it has been imperative to cope with such a situation also in the automotive industry. Under the circumstances, there have been proposed, as the countermeasure, for example, many technologies by which the efficiency of an internal combustion engine is to be enhanced to the utmost extent.
One of those technologies is that the efficiency of the internal combustion engine is maximized by subjecting, while detecting a combustion condition of the internal combustion engine, the internal combustion engine to feedback control according to the detected combustion condition. As a measure for detecting the combustion condition of the internal combustion engine, there has been generally known a technology in which ions generated by combustion of combustible gas mixture are detected as an ion current by an ion sensor, and it is determined whether or not the internal combustion engine is in the combustion condition, based on the detected ion current.
In this technology, the ion current very delicately changes according to a change in combustion condition of the internal combustion engine and various environments, and the change amount of a signal is large. Accordingly, in a signal processing device that retrieves an ion current signal into a microcomputer, for example, through an analog to digital (A/D) converter, and digitally processes the ion current signal, there arise such problems that a large signal is lost by the limit of a dynamic range of the A/D converter, a small signal is lost by the limit of a resolution of the A/D converter, and a fine change is not captured.
Under the circumstances, in order to solve the above-mentioned problems, there has been known provision of a sensitivity adjustment part for converting the signal to an appropriate magnitude in an interface part for retrieving the ion current signal in the signal processing device. In general, the sensitivity adjustment part is provided not only in the signal processing device for processing the ion current signal, but also in other signal processing devices in the same manner.
As the sensitivity adjustment part for converting the ion current signal to the appropriate magnitude to adjust the receiver sensitivity, a sensitivity adjusting part including two resistors and one switching element has been known up to now (see, for example, Japanese Patent Application Laid-open No. 2008-248831). The sensitivity adjustment part switches the switching element to select any resistor through which the ion current signal is to be transmitted so as to convert the magnitude of the ion current signal for adjustment of the receiver sensitivity.
In the sensitivity adjustment part of the related art, unless the switching element normally operates, the receiver sensitivity of the ion current signal cannot be adjusted. For that reason, there is conceivably a case where, even if the receiver sensitivity of the ion current signal is intended to be adjusted by the signal processing device, the receiver sensitivity is not actually adjusted, and as a result, appropriate signal processing cannot be executed. Under the circumstances, there is a need to diagnose whether or not the switching element of the sensitivity adjustment part normally operates to adjust the receiver sensitivity of the ion current signal to a desired sensitivity. That is, there is a need to diagnose whether or not the sensitivity adjustment part is in failure.
As a measure for diagnosing whether or not the sensitivity adjustment part is in failure, there has been generally known a measure for inputting a known signal to the sensitivity adjustment part at a specific timing, and confirming whether or not a signal output from the sensitivity adjustment part reaches to a desired value. However, this measure cannot be applied to the signal processing device that always executes signal processing. That is, there arises such a problem that this measure cannot be applied to the signal processing device that cannot take timing at which the known signal is input to the sensitivity adjustment part, that is, that cannot perform interruption for failure diagnosis processing.